Dynamics of calcium regulation in muscle--the key to contractile activation--are controlled by membrane potential and are influenced by chemical agents such as caffeine, procaine, and Dantrolene. Understanding has been hindered by inability in many cases to achieve rapid changes in potential or intracellular concentration of agents. The internal processes are so fast that one rate-limiting step has been a) for multicellular preparations such as heart, diffusion in the interstitial space; b) for single intact cells, the rate of depolarization, often limited by diffusion in the transverse tubular system in the attempt to activate the entire length of the cell by K depolarization; c) for fibers free of sarcolemma, diffusion through the unstirred matrix of contractile proteins and membrane systems. A newly developed microwave temperature-jump system will be used for the study of rapid-heating relaxation of single frog striated muscle cells and heart muscle bundles. Digital averaging and repetitive pulsing will be used to increase resolution. This technique will be used in studying contractile mechanisms and alterations of the calcium regulatory systems involved in excitation-contraction coupling in denervated muscle and in atrial trabeculae. The system will be particularly advantageous in the investigation of the role of length-dependent activation in Starling's Law of the Heart. It is also of great potential significance for the study of secretory cells.